Incorporated by reference herein is the Xicor xe2x80x9cSmart Battery Fuel Gauge/Safety Device For Cellular Phone Packsxe2x80x9d technical reference, U.S. patent application Ser. No. 09/620,308, entitled xe2x80x9cSerial Interface for a Battery Management System,xe2x80x9d assigned to the assignee of the present invention, and U.S. patent application Ser. No. 09/620,089, entitled xe2x80x9cBattery Management System,xe2x80x9d assigned to the assignee of the present invention. Xicor, Inc. is located at 1511 Buckeye Drive, Milpitas, Calif. 95035.
The present invention relates generally to electronic devices and more particularly to a method and apparatus for monitoring the charging and discharging of a battery.
Rechargeable batteries are used in many applications to power a variety of devices. Different devices will discharge rechargeable batteries at different rates depending on the function being performed by the device and corresponding load being applied across the battery terminals. For example, a portable computer may discharge a rechargeable battery quickly when computing complex graphic calculations on a processor and rendering a graphic image on a display. The same portable computer may discharge the rechargeable battery more slowly when it is placed in xe2x80x9cstand-by modexe2x80x9d and operation of the computer is temporarily suspended. Even when the portable computer is turned off, the rechargeable battery may also continue to discharge a small amount of current over time due to the internal resistance present in the battery.
Generally, the rechargeable battery is charged with a transformer that converts current from a conventional electrical outlet or automobile lighter into direct current suitable for charging the battery. Once the rechargeable battery reaches a maximum voltage, it is fully charged. To protect both the rechargeable battery and the electronic device that it powers, it is important to carefully monitor and control the charging and discharging processes. Specifically, a battery can overheat and be damaged during the charge cycle if it is charged beyond the specified battery capacity. Overcharging can also harm the electronic device as well as people handling the device if the battery leaks or is damaged. In the discharge cycle, for example, an electronic device may be damaged if a short develops within the battery or the device and the sudden increase in current causes the battery or device to overheat or melt.
The device used to measure the charge/discharge state of a battery is popularly called a xe2x80x9cgas gauge.xe2x80x9d Like the gas gauge on an automobile, the battery gas gauge measures how much charge is stored in a battery. Conventional gas gauge devices measure the current flow into and out of the rechargeable battery to measure the battery""s charge. These conventional gas gauges detect the current flow using a fixed resistor coupled in series between the battery and the load. The voltage drop across the series resistor is directly proportional to the current flow measurements into or out of the rechargeable battery. Unfortunately, this series resistor, though typically very small in size, consumes a significant portion of the available power delivered by the rechargeable battery over time. Moreover, a small series resistor cannot be used to accurately detect the wide range of currents drawn by many of the electronic devices. That is, the voltage drop produced by the very small series resistor may only be accurately detected when the current flow is high. If the current flow is low, most conventional gas gauges may inaccurately measure the very small voltage drop across this very small resistor. For example, the conventional gas gauge may not accurately detect the lower current used when a computer is placed in xe2x80x9cstand-byxe2x80x9d mode. Although the series resistor size can be increased to increase measurement accuracy, the larger series resistor will also increase the power lost across the series resistor and, at high currents, further reduce the voltage available to drive the load.
Conventional gas gauges also have difficulty determining the battery charge when a battery is used over long periods of time. These gas gauge devices must keep an accurate time base to integrate the current charge and discharge over time and determine the remaining battery charge. Consequently, accurate battery charge measurement depends on how accurately a conventional gas gauge measures elapsed time over several days or, in some cases, several months of battery usage. Keeping an accurate time basis generally requires additional circuitry and added complexity in the design of the gas gauge.
Even if battery charge and other information related to charging a battery were available it is difficult to communicate these facts with other devices. The battery and charger typically cannot communicate with other devices because there are no standards for such communication. Further, even with communication standards provided, they are difficult and expensive to implement for typical applications.
In one aspect of the invention, a charge measurement apparatus for use in a rechargeable battery powered system includes a bi-directional integrated power and sense field effect transistor (FET) having first and second power FETs operatively coupled to a rechargeable battery, a bias signal, a common node and a load and first and second sense FETs operatively coupled to provide mirror charge and discharge currents, a gas gauge circuit that uses the mirror charge current to measure a total charge into the rechargeable battery, uses the mirror discharge current to measure a total discharge from the rechargeable battery and determines the charge level in the rechargeable battery based on the total charge and the total discharge.
Another aspect of the invention includes a charge measurement apparatus for a rechargeable battery powered system including an integrated power and sense device having a power device and a sense device, connected to the rechargeable battery and a load; the sense device operatively coupled to provide a mirror current that is a fixed ratio of the current in the power device, and a first circuit that uses the mirror current to measure the charge in the rechargeable battery.
Yet another aspect of the invention includes a charge measurement apparatus for a rechargeable battery powered system including a bi-directional integrated power and sense device connected to the rechargeable battery and a load, wherein bi-directional integrated power and sense device includes first and second power devices and first and second sense devices, the first sense device operatively coupled to provide a mirror charge current which is a fixed ratio of the current in the first power device, the second sense device operatively coupled to provide a mirror discharge current which is a fixed ratio of the discharge current in the second power device, and a first circuit that uses the mirror charge and discharge currents to determine the charge into and discharge from the rechargeable battery.
In another aspect of the invention, a method of measuring current flow in a rechargeable battery includes providing a integrated power and sense device having a power device and a sense device, providing a mirror current from the sense device that is an accurate ratio of the current flowing in the power device and the rechargeable battery, and measuring the charge in the rechargeable battery using the mirror current.